Digital halftoning is a technique employing digital image processing to produce a halftone output image from a continuous-tone input image. In the digital halftoning technique, a continuous-tone image is sampled, for example with a scanner, the samples are digitized and stored in a computer. The digitized samples or pixels consist of binary representations of scanned gray level values typically ranging from 0 to 255. To reproduce this image on a printer capable of printing dots of only one gray level (e.g. black) it is necessary to create the sensation of gray levels by suitably distributing the printed dots in the output image. The continuous-tone input image is converted to a binary output image, that is a pattern of "1's" and "0's" where the "1's" represent a dot and the "0's", represent the absence of a dot to be produced by the output printer. One prior art method of digital halftoning, called error diffusion, distributes the dots in the output image by selecting for each continuous-tone input pixel a binary output pixel (or patterns of output pixels) according to an error criterion. After selecting the binary output value, the error between the continuous-tone input value and the binary output value is determined. This error is then propagated to neighboring pixels to be employed in the error criterion. A variety of methods have been employed for allocating the distribution of error to neighboring pixels. Most of these methods produce a visible artifact called "worms" in the image as a result of the propagation of the errors through the image as it is processed line by line. Most of the methods also reduce the resolution of the image.
U.S. Pat. No. 4,654,721 issued Mar. 31, 1987 to Goertzel et al. is directed to providing a solution for these problems in an error diffusion technique where each input pixel is represented by a block of output pixels. Goertzel et al. avoid the appearance of worms by employing a random number generator to determine the distribution of error between two adjacent blocks of pixels, such that a random fraction of error is distributed to one block and the remainder of the error is distributed to the other. In another technique sometimes called error diffusion with dither, the error criterion is randomly varied to reduce the appearance of worms.
Although the techniques described above reduce the appearance of worms they also increase the random noise in the output image. To the extent that this random noise occurs in the frequencies visible to the human viewer, the image is thereby degraded.
It is the object of the present invention to provide an improved error diffusion digital halftoning technique that avoids the problems noted above.